A conventional two-wheeled motor vehicle seat device is known in which, to the rear of a front seat for a driver a rear seat for another occupant is formed so as to be one step higher, a backrest for the driver to lean on is provided at the stepped border between the two seats, and the backrest can be adjusted by moving it in the fore-and-aft direction to thus change the seat position for the driver (see Patent Document 1 below).